Continuous steelmaking is particularly advantageous in those regions where there is a concentration of production, or ready availability of metallic charge, and where electric energy is both available and economical.
The operation of an electric arc steelmaking furnace can be an intermittent operation, wherein the sequence followed is: charging of steel scrap and/or direct reduced iron, pig iron, slag formers and alloying elements; ignition or establishment of an electric arc between the electrodes in the furnace to create melting conditions for melting the charge and forming a molten metal bath covered by a foamy slag; refining for a period of time during which the molten metal portion of the bath is refined to form steel having a desired composition and quality; and periodically raising the electrodes to remove them from contact with the bath and interference with the tapping procedure; and then tapping the molten metal.
In the steelmaking practice, "continuous charging" or "continuous melting" refer to charging practices in which charge materials are fed to a furnace during the charging, melting and refining periods, then charging and power input is interrupted for the tapping procedure. An electric arc steelmaking furnace can be operated continuously without interruption by charging or power input for the tapping procedure. The term "metallic charge" as used herein means material for continuous melting, including ferrous scrap, pig iron and direct reduced iron in pellet or briquette form. In continuous operation, metallic charge is preheated and continuously fed to the furnace. A foaming slag practice may be used so that the furnace is only partially tapped intermittently without removal of the electrodes. Thus, the electrodes remain at full power during continuous feeding, refining and tapping.
In the continuous steelmaking process, as well as in all steelmaking processes, control of the steel content is desirable to manufacture different compositions and qualities of steel. Control of carbon content and iron content in steel is typically accomplished by selecting specific grades of metallic charge to be fed into the furnace as well as by introduction of pure material.
Accurate measurement and control of the bath temperature is desirable to maintain optimum melting conditions as well as to use energy efficiently in the steelmaking process. Measuring the bath temperature on a periodic and continuous basis is important to avoid steel bath overcooling or overheating. Manual measurements of bath temperature may produce inaccurate and unreliable temperatures because of inconsistent placement of temperature measuring instruments. Although manual measurements of steel bath temperature can be used in the continuous steelmaking process, more accurate and reliable steel bath temperature measurements are preferred.
Providing a steel bath level that is neither too low nor too high is also important to maintain the steelmaking process in a continuous mode. In particular, maintaining an appropriate heel level (i.e., minimum level of molten metal in the furnace) containing available heat results in immediate melting of metallic charge. Accurate determination of the steel bath level further allows for proper placement of sensing equipment and material injectors in relation to the steel bath level.
Accurate measurement of the steel bath level allows for control of steel production. Measuring the steel bath level may be accomplished by visual inspection, or estimated based on the amount of metallic charge introduced to the furnace (i.e., the metallic charge feed rate). The metallic charge feed rate may be determined by: weighing each load of metallic charge; measuring the change in position of the loading crane along the charge conveyor; estimating a metallic charge velocity based on the change in position of the loading crane; and calculating the metallic charge feed rate based on the weighed metallic charge and the estimated metallic charge velocity. Although the metallic charge feed rate may be determined as described above, a more accurate and simpler determination of metallic charge feed rate is desirable. Determining accurate feed rate is particularly important to control the bath level in a continuous steelmaking process.
Although this invention is shown and described in connection with an electric arc steelmaking furnace, it will be readily apparent that any electric powered steelmaking furnace, including, but without limitation, plasma furnaces and induction furnaces, could be substituted for the electric arc steelmaking furnace with like results.
The method and apparatus of the present invention is particularly well suited for use in the continuous steelmaking process described in my U.S. Pat. Nos. 4,543,124 and 4,609,400.